One type of prior offshore system for transferring hydrocarbons between an underwater pipe and a vessel uses a heavY duty riser to moor the vessel to limit drift while oil is transferred through a separate conduit. U.S. Pat. No. 3,979,785 by Flory shows one system of the this type, wherein the riser is a heavy duty chain whose lower end is anchored by a chain table held by catenary chains and whose upper end is held by a buoy which moors a ship. In that system a separate flexible hose extends from near the bottom of the riser along a separate path to the ship. Another approach shown in U.S. Pat. No. 4,490,121 by Coppens shows a heavy duty riser in the form of a large diameter pipe or body with its upper end supported by the bow of a vessel and its lower end anchored by catenary chains, to apply large forces that moor a large tanker. Fluid is carried by hoses that extend through the hollow body, with the hollow body carrying substantially all tension passed along the riser. These systems are heavy and costly because they must hold a large ship in position.
Lower cost fluid transfer mooring terminals can sometimes be constructed by using a dYnamically positioned vessel which is connected through a neutrally buoyant hose to a pipe at the sea floor. The DP vessel (dynamically positioned vessel) may use a wire line reference system (a wire extending from the sea floor to the ship, whose angle indicates drift) to monitor vessel drift so a propulsion system on the vessel can move it to avoid excessive drift that would harm the hose. However, the position of the flexible hose is largely uncontrolled, so it may become damaged and there would be interference between the wire line and hose. Such interference is also likely if the vessel is allowed to revolve in a weathervaning mode to reduce propulsion power. A fluid transfer system for use with a dynamically positioned vessel, which enabled control of hose position in a low-cost transfer system, and which facilitated measurement of vessel drift without the need for a separate wire line, would be of considerable value.
The thruster equipment of a DP vessel has a limited lifetime of use (before overhaul is required), with the lifetime dependent on the period during which it is operated at more than verY low power (used to lubricate the bearings). This is a disadvantage in production from an undersea well, as production systems are generally costly to disconnect from and reconnect to. A production system which avoided the limited lifetime of often-used thruster equipment while avoiding the cost of a heavy passive mooring system, would be of considerable value.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, an offshore fluid transfer system is provided for transferring fluid through a conduit between an underwater pipe and a dynamically positioned vessel, which enables the simple determination of vessel drift, and which enables the system to be constructed at moderate cost. The system can comprise a mooring system which has sufficient strength to moor the vessel under calm to somewhat turbulent seas, but insufficient strength to moor the vessel in stormy seas. In stormy seas the propulsion system of the dynamically positioned vessel serves to limit vessel drift. The lifetime of maintenance-free use of the propulsion system is greatly extended by the fact that it operates only once in a while at moderate to high power levels.
The system can include a riser having an upper end pivotably attached to the dynamically positioned vessel and a lower end with a chain table held by catenary chains extending to the sea floor. The lower portion of the riser is weighted, and substantially all of the weight is supported by tension in the riser, which maintains the riser largely straight. A lightweight riser can be formed by one or a few conduits, which may be a flexible hose, extending most of the length of the riser, with the conduit maintained substantially straight by carrying the moderate tension of the riser.
Drift of the vessel can be determined by measuring tilting of the upper end of the riser. The pivotal mounting of the riser upper end to the vessel may be through a universal joint, and pivoting of parts of the universal joint can indicate tilting of the upper portion of the riser. Where most of the riser length is taken up by a flexible hose, the top of the riser may include a rigid pipe extending a plurality of meters, so tilt of the rigid pipe mcre closely represents average tilt of the entire riser.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.